This invention relates generally to switching transistor circuits, and more particularly to an improved inductively loaded switching transistor circuit with automatic base current control.
In the case of portable communication systems such as selective call receivers or pagers, battery life is one of the key parameters which will ultimately determine the success or failure of a product in the marketplace. It is therefore essential that current drain be minimized.
Certain radio pagers known in the prior art utilize a DC-DC switching regulator power supply to generate a voltage B.sup.++ from a single cell battery (B.sup.+). U.S. Pat. No. 4,355,277 entitled "Dual Mode DC/DC Converter" describes a DC-DC converter circuit wherein the base drive current for the switching transistor in each mode must be set to a worst case (high) value in order to assure performance under all circumstances which may vary due to variations in B.sup.+, coil resistance, B.sup.++ load current, etc. This results in wasted current which reduces circuit efficiency thus negatively impacting product battery life.
U.S. Pat. No. 4,056,734 entitled "Compensated Base Drive Circuit to Regulate Saturated Transistor Current Gain" described a single-ended self-oscillating DC-DC converter which includes base drive compensation circuitry. The compensation circuitry is active during saturation intervals of the power transistor to control its forced current gain to some fixed value irrespective of the magnitude of the saturated current flowing through the transistor. Unfortunately, this circuit is not optimized for minimum current drain since base current is only diverted from the base of the switching transistor.